PTFE was considered almost intractable in the early years requiring high performing pressure and complex fabricating methods. Consequently, much lower molecular weight melt processable resins FEP, PFA, ETFE as well as other melt processables were developed. Melt processable types have not fulfilled the needs hoped for PTFE resins. These resins are more expensive and provide new fabrication problems which prohibit their use to produce and form large parts and sheet in a wide range of thicknesses, widths, and lengths. Filled and truly reinforce compositions are impossible to produce from melt processable fluoropolymer resins. Melt process resins did fill a nitch but did not live up to the expectations for PTFE resins.
Since the discovery of PTFE (also known by the tradename “TEFLON”) in April of 1938 by Plunkett of DuPont, methods of fabrication have slowly developed due to the unfamiliar polymeric properties of this new material. The extremely high molecular weight and lack of perceptible melt flow of PTFE resin relative to other well-known plastic materials is blamed for PTFE's slow growth. The unfamiliar behavior of PTFE resin as a melt-processable plastic material forced fabricators to look elsewhere for help. New fabrication techniques gradually developed around methods employed for processing powdered ceramics and metals. This trend continued well into the 1950s and 1960s until melt-processable resins were developed and became commercially available with high hopes of solving PTFE resin problems. The molecular weight of these resin types had to be drastically lower to accomplish the desired melt flow for melt extrusion.
Most of the beneficial virtues of higher molecular weight PTFE resins were not found in the melt flow resins developed. It was also discovered that melt process resins do not lend themselves to compounding with fillers and reinforcements. The following art processes for PTFE resin have emerged over the years. Art Skived Sheet Processing.
A virgin PTFE sheet is currently produced by shaving (skiving) a compression-molded cylinder of sintered granular resin held in a lathe. This is done much the same as wood is shaved in the manufacture of plywood. A manufacturing problem arises because of the massive size of the required molded cylinder (billet). Fluoropolymers, such as PTFE, all have a very narrow safe range for melting and sintering. Above the upper safe limit, the PTFE polymer degrades very rapidly and decomposition accelerates as the temperature exceeds that safe limit. In addition, all fluoropolymers possess very low thermal conductivity and require long sintering cycles to accomplish uniform fusion processing cannot be accelerated by raising the temperature. Thermal degradation frequently occurs because of the low thermal conductivity of the PTFE and the lack of needed temperature control during the long sintering cycles required. Even if heating is well controlled, too rapid heating or cooling may result in a cracked billet if thermal expansion and contraction is not uniform. Large billets are sintered standing free, the pseudo-melt is a very stiff gel which if not heated uniformly will sag and may crack. Sintering cycles often require a full day or more to uniformly control the thermal changes. The density of the sintered billet may vary widely from inside to outside as well as from end to end. The variations in density which occur are reflected in the skived sheet's dimensional stability and may cause the sheet to warp so it will not lay flat. The skived sheet retains the memory of its origin and history in the sintered billet; the result is a sort of sine wave in the surface of the sheet when an attempt is made to lay the sheet flat. In order to obtain a flat sheet, the sheet must be subjected to reheating above its remelting point, of 327 degrees Centigrade, to recrystallize the resin and equalize sheet density and thus remove the retained warp and stress held in the sheet. To accomplish flatness the sheet is confined between metal plates and re-sintered above its melt transition temperature (this requires large well controlled ovens). The process for obtaining useable flat sheet as well as billet molding is time and energy intensive, Waste is of the order of 10 to 15 percent (10 to 15%) in trimmings from the ends of the billet and polymer adjacent to the skiving mandrel, etc.
The molding billet process for making PTFE filled sheets has proven to be impractical for many reasons. The molding and sintering steps must be performed within the confines of the billet mold under high pressure (5 to 10,000 plus, PSI). A quality filled composition above 30 percent (30%) by weight is not commercially available. Dulling of the skiving blade by the fillers becomes a major problem. Only granular molding grade PTFE is usable in the billet molding process; coagulated dispersion resin cannot be processed without cracking during sintering.
An Improved Art Molding Resin
PTFE granular resins have been excluded from art processing because all available resins are anisotropic. The polymerized particle is a heterogeneous spongy contiguous construction roughly 300-400 microns average particle size and not workable without particle size changes. When comminuted, to change the above particle size the particle may be reduced to approximately 50 microns plus, yielding a substantial portion of mechanically-produced anisotropic resin fibers as disclosed in U.S. Pat. No. 2,936,301, issued to Thomas, et al. on May 10, 1960. The latter product can be converted to a pellet form by a process disclosed in U.S. Pat. No. 3,766,133, issued to Roberts et al. on Oct. 16, 1973, and marketed by DuPont under the name TEFLON 7. The fibrous particles are anisotropic and display some dimensional instability therefore are not used to produce sheet because preforms are weak and fragile. TEFLON 7 is used in its pelletized form primarily for automatic molding of small parts because of its excellent flow as a pellet.
Biaxial Planar Oriented Sheet.
A method for manufacturing biaxially-oriented structures, such as sheets, was disclosed in Roberts U.S. Pat. No. 3,556,161 known as the biaxial calendering method. This method involves the application of multiple biaxial calendering passes involving concurrent compressive shear stresses to PTFE dispersion resin particles in 16 to 20% wetting liquid. The application of compressive and shear stress components in processing are directed so that the component vectors result in a biaxially planar oriented structured product. The product has excellent properties but limits size and requires multiple processing steps.
Paper Making
The process described in this invention has many of the mechanical characteristics of paper-making. However, paper-making starting materials are usually cellulosic fibers or similar materials processed in a water medium. Fibers made from wood pulp must be pre-processed from the solid timber to become free fibers. The wood is reduced to a pulp by a comminuting and beating process that frees the fibrous material. The reduction to pulp and the further processing is all processed in a water medium. Historically polytetrafluoroethylene resin has been manufactured in particulate form to deliberately avoid any tendency to produce fibers. This was done because all of the automatic methods of processing employed in industry require symmetrical particles with good handling characteristics, namely to be free-flowing and capable of leveling uniformly where a shallow sheet mold is required. Fluoropolymer manufacturers felt that the only way to produce a quality sheet product would depend upon the development of a melt-processable resin type.
Two attempts were made to paper-make coagulated dispersion polymer, disclosed in U.S. Pat. No. 3,003,912, issued to Harford on Oct. 10, 1961, and U.S. Pat. No. 3,010,950, issued to Thomas on Nov. 28, 1961. Both methods attempted to prepare PTFE from coagulated dispersion resin fibers suitable for calendering into sheet. Harford produced processable fibers by paste extruding coagulated dispersion powder lubricated with 20 percent (20%), “Skellysolve E” (a petroleum fraction) to produce a rod containing fibered polytetrafluoroethylene. After the one-eighth inch (⅛″) diameter rod was dried, it was cut into one quarter (¼ to 1″) lengths. It was found that by rubbing rod segments together vigorously in a micro-pulverizer or hammer mill that the segments would shred to produce a fibered composition. The fibers thus extracted were processed in a water medium according to customary paper-making art. When the felt-like product produced was fused by sintering at 350 to 370 degrees Centigrade, the sheet shrank to 41 percent (41%), of its previous area prior to sintering. The product produced was found to be air permeable and similar to paper. These fibers and the processed fibers were anisotropic, they shrank.
Thomas describes a process in which coagulated dispersion resin particles are “water-cut” in a high speed bladed cutter in water. Cutting is continued until a major portion of the particles are deformed into what is described as “bola-shaped” particles. The powder produced above, according to the teachings, can be calendered into sheet. This patent claims only a polytetrafluoroethylene fine powder form. Both of the patents utilize water as a processing medium. Water is hydrophobic to fluoropolymers and will cause the resin to clump or aggregate. The fact that water does not wet fluoropolymers hinders processing and the forming of pore-free structures. A quality product was never produced utilizing the Harford or Thomas methods. Both products were anisotropic.
Particulate Characteristics Make A Difference.
Particulate differences significantly influence the processing and performance of product. Isotropic fibers are an important aspect of the present invention. Particulate matter is comprised of molecules which are arranged to define the particle shape; essentially, as spheres, plates or fibers. The molecular orientation of the molecules within the particle may influence processing and product performance. X-ray diffraction, infrared spectroscopy and tensile properties may be utilized to determine these molecular structure differences. Polytetrafluoroethylene resin is one of the longest linear chain molecular polymers and is also very nearly 100 percent (100%) crystalline as polymerized and unmelted; it is and therefore ideal for analysis. (See Roberts U.S. Pat. No. 3,556,161).
The properties most important dimensionally to processing and product performance are particle size, shape and size distribution; the degree and direction of molecular orientation within each particulate form influences the performance of the fabricated product.
In this invention resin particles and micro-fibers are isotropic; they are form stable and uninfluenced by processing and fabrication. In contrast art resin particles and micro-fibers are anisotropic; product is influenced dimensionally particularly when heated or fused.
Isotropic vs. Anisotropic Properties
These terms are employed to define the dimensional stability of particles and fibers. Art fibered resin and their products are unstable dimensionally, therefore termed anisotropic. For example, fibers produced from paste extruded dispersion resin by Harford U.S. Pat. No. 3,003,912 (referred to on page 7) fabricate into sheet by papermaking methods, in a water medium, produced a porous sheet which shrink 41% in area on sintering, (see column 3, lines 5-6 of the Harford patent). The shrinkage and lack of resin cohesion inhibited resin fusion which is typical of anisotropic product.
In contrast, fibers processed by the present invention from the same dispersion type resin and deposited in the same manner as Harford produces a void free sheet product that is form stable and will lay flat. This product is isotropic.
Granular Compression Molding Resin.
All compression molding resins named in the trade “granular resins” are anisotropic and perform as such. Anisotropy is one of the reasons for poor preform strength, and fragility.
The next most important factor for poor preform strength and poor fusion on sintering is the available particle size, shape and size distribution of PTFE particles available in art granular resins; the as polymerized morphology of granular resin is large; necessitating comminuting. The smallest particles produced are of the order of 50 microns; these particles do not flow or handle well.
The process of the present invention provides the only known method of preparing microscopic size isotropic PTFE particles and also provides a convenient means of handling these microscopic particles, as well as co-processed fibered reinforcements and additives during processing. Colloidal (dispersion size particles) PTFE resin particles are the raw material of choice required by this invention not granular PTFE.
There are the fundamental differences between granular and coagulated dispersion resin, the latter being the source of the colloidal particles for the present invention.
The highest molecular weight PTFE polymer is granular resin used almost exclusively for compression molding; it is the oldest segment of PTFE technology. Coagulated dispersion resin followed years later and has never been seriously considered for anything resembling the processes used for granular resin.
Granular resin is used for automatic compression molding of small parts (rings, washers, gaskets, etc.) and relatively thick cross section parts; cylinders for skiving (shaving) sheet, the only source of PTFE sheet (from several mils thick to ⅛ inch); also used for molding nose cones for missiles and isostatic molding of pipe and fillings. Skived sheet lacks quality without added finishing to relieve stress and permit the sheet to lay flat. Sheet also lacks the length and width required for use in chemical linings.
Automatic and isostatic molding methods were promoted after particle flow problems were solved in the 1960s. Performing pressures greater than 2,000 psi are required to produce low void content sheet. A granular molding resin, suitable for sheet molding, has not been available because low pressure performing and good leveling properties could not be developed. The large size and shape of resin particles as well as anisotropic behavior of available resins promotes poor particle fusion.
An acceptable granular resin was developed in the mid 1950s that had reasonable moldability and handling by Thomas in U.S. Pat. No. 2,936,301 and Roberts et al. in U.S. Pat. No. 3,766,133. Thomas produced a resin with controllable anisotropy that was somewhat fibrous but did not have the desired flow. Roberts et al. devised a method of reconstituting Thomas's fibered resin to produce symmetrical pellets with excellent flow properties and good moldability. This resin form however could not be used to fabricate sheet because of a lack of adequate resin cohesion which contributes to preform fragility, typically produced by anisotropic particles. The resin did handle and flow well where preform pressures above 2000 psi could be developed; however preforms were still fragile and required special handling.